In Homogenous Charge Compression Ignition (HCCI) operation of internal combustion engines, the temperature of the charge for the combustion should not be too low. If the temperature is too low, the combustion will be phased lately, resulting in a lower efficiency, and if even later also increased carbon oxide (CO) and hydro carbon (HC) emissions. At an even lower temperature, the combustion will fail to appear, i.e. there will be a misfire, resulting in decreased comfort for persons in a vehicle in which the engine is operating.
In order to solve this problem, U.S. Pat. No. 7,194,996B2 suggests in a direct fuel injection engine during HCCI operation performing a pilot fuel injection during a negative valve overlap at an exhaust stroke and an intake stroke of the piston, in order for the pilot fuel to react with residuals from a previous combustion. Subsequently, air and additional fuel are introduced during the intake stroke and/or a compression stroke for the main combustion. The purpose of the pilot fuel and residual reaction is to heat the charge for the subsequent main combustion. The heated charge will prevent the main combustion from occurring too late.
Although this known method has proven advantageous, there is still use for a solution increasing the combustion control at HCCI operation. For example, a reaction with the fuel from the pilot injection requires a presence of oxygen in the residuals, and therefore the preceding main combustion has to take place with a lean mixture so that there is a surplus of air, some of which is retained during the negative valve overlap. However, the inventors have discovered that an excess of pilot fuel, providing a rich mixture with the air in the residuals, has a cooling effect on the charge for the subsequent main combustion.
Thus, in one embodiment, a method for an engine system is provided, the engine system including an internal combustion engine having at least one cylinder at which a piston, at least one inlet valve, at least one exhaust valve, and a fuel injector for injection of fuel directly into the cylinder. The method comprises performing the following in at least one of the at least one cylinder:                controlling at least one of the at least one intake valve so as to introduce air into the cylinder,        performing at least one main combustion fuel injection for a main combustion with air introduced into the cylinder,        controlling, during an exhaust stroke and an intake stroke, the intake and exhaust valves so as to form a negative valve overlap to capture main combustion residues, and        performing at least one pilot fuel injection during the negative valve overlap; and        where an amount of fuel in the at least one pilot fuel injection is at least partly dependent on said introduction of air and at least one of the at least one main combustion fuel injection        
In this way, it is possible to ensure that the pilot fuel amount is not in excess in view of the oxygen amount in the residuals, in order to avoid un-oxidized pilot fuel to cool down the charge for the next main combustion. Thus, it is possible to improve combustion stability at homogenous charge compression ignition operation of an internal combustion engine. Further, it is also possible to increase efficiency at homogenous charge compression ignition operation of an internal combustion engine. Further still, it is possible to decrease emissions at homogenous charge compression ignition operation of an internal combustion engine. Finally, it is also possible to to reduce risks of discomfort for persons in a vehicle in which the engine is operating with homogenous charge compression ignition.
Also, by the above operation it can be ensured that enough pilot fuel is injected in view of the oxygen amount in the residuals in order to reach enough heating of the charge for the main combustion.
A late main combustion at HCCI operation will reduce the combustion efficiency and the thermodynamic efficiency. By avoiding cooling of the main combustion charge, risks of a late main combustion are reduced, which will increase engine efficiency, reduce fuel consumption, and reduce risks of increased CO and HC emissions. Also, passenger comfort in a vehicle in which the engine is operating is secured, since risks of misfire will be reduced.
Regarding the different steps of the example method, the step of performing at least one main combustion fuel injection for a main combustion could be done by performing the at least one main combustion fuel injection during an intake stroke and/or during a compression stroke of the piston. The step of controlling the intake and exhaust valves to form a negative valve overlap to capture residues from the main combustion, can be carried out by closing the exhaust valve during an exhaust stroke of the piston following immediately upon a power stroke of the piston, in turn following immediately upon said compression stroke, and opening the intake valve during an intake stroke of the piston following immediately upon said exhaust stroke, to form a negative valve overlap between said exhaust valve closing and said intake valve opening.
It should be noted that the intake stroke and the power stroke are defined as periods during which the piston moves downward from the top dead centre (TDC) position to the bottom dead centre (BDC) position, regardless of positions of the inlet valve(s) and the exhaust valve(s) during this downward movement. Similarly, the exhaust stroke and the compression stroke are defined as periods during which the piston moves upward from the BDC position to the TDC position, regardless of positions of the inlet valve(s) and the exhaust valve(s) during this upward movement.
In another example, the method may comprise:                introducing a first amount of air into the cylinder,        performing the at least one main combustion fuel injection for a main combustion with air in the first amount of air,        capturing, during the negative valve overlap, residues from the main combustion, and        determining the amount of fuel in the at least one pilot fuel injection at least partly based on said first amount of air and the amount of fuel injected in at least one of the at least one main combustion fuel injection.        
This means that each pilot fuel can be determined at least partly based on the amount of fuel injected and the amount of air introduced for the main combustion preceding the pilot fuel injection, which will result in a very accurate fuel amount determination.
The first amount of air can be introduced into the cylinder by controlling the intake valve so as to be open during at least a part of an intake stroke and/or during a part of a compression stroke of the piston. The introduced amount of air can be the amount of air allowed into the cylinder, or the amount of air captured in the cylinder. If the intake valve closes in the compression stroke, some air allowed into the cylinder may be pushed back into the air intake duct, and thereby the mount of air allowed into the cylinder and the amount of air captured in the cylinder would differ. Which of these amounts correspond to the amount of air introduced into the cylinder, as stated in claim 1, would depend on the manner in which air transportation to the cylinder is determined. The introduced amount of air may be regarded as the amount of air captured in the cylinder.
In still another alternative, the method comprises:                determining a flow of air into the cylinder,        determining a flow of fuel for a plurality of main combustion fuel injections for a plurality of main combustions with air introduced into the cylinder, and        determining the amount of fuel in the at least one pilot fuel injection (P1), at least partly based on the flow of air and the flow of fuel for the plurality of main combustion fuel injections (P21, P22).        
For example, the amount of fuel in the at least one pilot fuel injection can be determined based on a flow of fuel for a plurality of pilot fuel injections, in turn determined at least partly based on the flow of air and the flow of fuel for the plurality of main combustion fuel injections.
In the art, the ratio, between the actual air/fuel ratio and the stoichiometric air fuel ratio, is referred to as the lambda value. The amount of fuel in the at least one pilot fuel injection may be such that the lambda value of a charge with the amount of fuel in the at least one pilot fuel injection and a second amount of air in the residues from the main combustion captured in the negative valve overlap is higher than a predetermined lambda threshold value. In one example, the predetermined lambda threshold value is 1. Thereby, it is ensured that substantially all pilot fuel will be consumed in a reaction with oxygen in the residuals, so that un-oxidized pilot fuel will not cool down the charge for the next main combustion.
The amount of fuel in the at least one pilot fuel injection may be such that said lambda value is within the range of 1.0-1.6. Keeping the lambda value within this range will secure avoiding said cooling effect and still provide enough pilot fuel to obtain a reaction with residual oxygen for a sufficient heating of the main combustion charge.
Additionally, one of the at least one main combustion fuel injection is a first main combustion fuel injection and another of the at least one main combustion fuel injection is a second main combustion fuel injection, the amount of fuel in the at least one pilot fuel injection being determined partly based on the first and second main combustion fuel injections. Thereby, where there are a plurality of main combustion fuel injections for each main combustion, based on the amount of air and the total amount of fuel injected for the previous main combustion, it is possible to properly determine the amount of oxygen in the trapped residuals at the pilot injection. As exemplified below, the first main combustion fuel injection can be a main fuel injection effected at the intake stroke during the negative valve overlap and after a reaction between a previous pilot injection and residual oxygen, and the second main combustion fuel injection can be a post fuel injection effected during the subsequent compression stroke shortly before the main combustion.